mysql的auto_increment注意点

mysql的5.6.27版本修复了一些bugs，其中有一个bug是关于auto_increment的，链接地址在 http://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-27.html，
第一条bug fixed就是

InnoDB: Reloading a table that was evicted while empty caused an AUTO_INCREMENT value to be reset. (Bug #21454472, Bug #77743)，但是并没有解决数据库重启后空表的auto_increment被重置的场景

一、场景重现：

[root@Kenyon ~]# mysql test
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.6.27-log Source distribution

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owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> create table tbl_kenyon(id int auto_increment primary key,vname varchar(32)) auto_increment 1000 engine innodb;
Query OK, 0 rows affected (0.16 sec)

mysql> create table tbl_kenyon2(id int auto_increment primary key,vname varchar(32)) auto_increment 1000 engine innodb;
Query OK, 0 rows affected (0.04 sec)

mysql> insert into tbl_kenyon (vname) values('Just test');
Query OK, 1 row affected (0.03 sec)

mysql> show create table tbl_kenyon \G
*************************** 1. row ***************************
       Table: tbl_kenyon
Create Table: CREATE TABLE `tbl_kenyon` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `vname` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> show create table tbl_kenyon2 \G
*************************** 1. row ***************************
       Table: tbl_kenyon2
Create Table: CREATE TABLE `tbl_kenyon2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `vname` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1000 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

--重启数据库后，可以看到auto_increment变化了

mysql> show create table tbl_kenyon\G
*************************** 1. row ***************************
       Table: tbl_kenyon
Create Table: CREATE TABLE `tbl_kenyon` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `vname` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> show create table tbl_kenyon2\G
*************************** 1. row ***************************
       Table: tbl_kenyon2
Create Table: CREATE TABLE `tbl_kenyon2` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `vname` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

二、分析
innodb的auto_increment值并没有固化下来，而是在内存中，假如重启了db，那么会被重置为当前表中最大的自增值，myisam的不受此影响，因为值是存在.MYI文件里的。早期的版本中可能还有另一种奇葩的情况是建一个空表，alter表重置auto_increment，不去操作过一段时间后自己会被重置成1，不过这个bug官方给出的答复是在5.6.27被修复了。

三、影响

1.对自增值有要求的场景存在风险
2.与历史数据合并时可能存在主键冲突

四、参考：
1.http://bugs.mysql.com/bug.php?id=78491
2.https://bugs.mysql.com/bug.php?id=77743